1. Field of the Invention
The invention relates to aircraft control systems and more particularly to a system for exiting a windshear condition in a manner that maximizes the distance traveled and the time the aircraft remains in the air.
2. Description of the Prior Art
Windshear, encountered on takeoff or landing, can pose a serious threat to the safety of aircraft and has been attributed to several aircraft accidents, including the recent crash of an L-1011 aircraft at Dallas, Tex. on Aug. 2, 1985. Windshear has been defined as a rapidly changing horizontal wind or a steady vertical wind whose effect on the aircraft is to cause large speed or altitude deviations from normal flight. Either as a direct result of loss of airspeed and altitude induced by the windshear or as a result of maneuvers by the human pilot to restore the aircraft to its normal flight path, windshear can cause the aircraft to stall or crash.
Prior art systems have included means for detecting and measuring the magnitude of the windshear and or providing guidance to the human pilot or autopilot which would cause the aircraft to fly at some fixed speed, usually slightly greater than stall speed. The speed commanded was usually a speed known as stick shaker speed, which is approximately 5% greater than stall speed, and is the speed where artificial means are coupled to the control column or stick to cause a vibration and warn the human pilot of impending stall. Stick shaker speed has generally been considered to be the minimum speed for safe flight. Corresponding to stick shaker speed is a stick shaker angle of attack, which is generally considered to be the maximum allowable angle for attack for safe flight of the aircraft.
Since may commercial transport aircraft, general aviation aircraft and military aircraft are equipped with a flight director system whereby pitch command signals may be displayed to the human pilot, the guidance command for a windshear encounter is usually presented as a displacement of the pitch command bar. When the human pilot maneuvers the aircraft in such a manner as to reduce the displacement to a null value, he has assured that the aircraft is at the required pitch angle to satisfy the guidance command. In addition, many aircraft are also equipped with an automatic pilot system which can be coupled to manipulate the elevator control surface of an aircraft in order to respond to a predetermined guidance control law, such as one which might be used to command the aircraft to the optimum flight path in the event of a windshear encounter.
A shortcoming of the prior art is that the command fixed speed or angle of attack may result in causing the aircraft to fly at the minimum safe speed when the magnitude and the duration of the windshear do not in fact require such a maneuver. In addition, a command to fly at the maximum angle of attack can excite the phugoid mode of oscillation, which is a long, poorly damped oscillation of the aircraft involving changes of speed and altitude with a period that may exceed two minutes for a large airplane. Excitation of the phugoid mode can result in loss of control and a crash of the aircraft even after the windshear condition has abated. Consequently, prior art systems could in fact create dangerous situations wherein the aircraft would crash even in the presence of relatively low magnitude shear.
Another prior art scheme is discussed in pending application Ser. No. 834,729, Flight Guidance for Aircraft Windshear, co-invented by one of the present inventors and assigned to the assignee of the present invention. In said application, a command was generated to reduce the aircraft's true airspeed at a rate proportional to the magnitude of the encountered windshear, rather than to a fixed airspeed. This control law effectively minimized the flight path angle change in a shear encounter and provided improved guidance commands, but did not adequately take into account the long term phugoid mode oscillations of the aircraft.
The present invention overcomes the limitations of the prior art by providing a guidance command that effectively minimizes excitation of the phugoid mode, while commanding a flight path angle at a minimum elevation sufficient to clear any obstacles, such as tall buildings or hills that may be found around airports and compensate for downbursts. The invention maximizes the time the aircraft remains in the air and the distance traveled, regardless of the magnitude of the windshear or whether the winds are horizontal, vertical, or a combination of the two.